One of the difficulties when constructing laser warning devices consists in the mastering of the great dynamic range of the incident light intensity. The range within which one could want the warner to function is within the irradiance extremes 10 mW/m.sup.2 and 10 GW/m.sup.2, i.e. the dynamic range amounts to 120 dB. If a range from 10 W/m.sup.2 to 1 GW/m.sup.2 is sufficient, the dynamic range will be 80 dB, which may be said to be a more reasonable value, but still a range that is technically difficult to measure. In addition to this difficulty, the illumination is in most cases pulsed and the pulses are very short (nanoseconds).
There are different ways of solving the problem. Signal processing that is based on the measuring of illumination-analog signals requires, for reasons of dynamics, a plurality of detectors and a plurality of signal paths. For each signal path, advanced electronics requiring great accuracy is necessary. If, on the other hand, one tries to build a signal receiver that functions digitally in the sense that illumination towards a detector produces a single type of response if the selected threshold value is exceeded, 360 detectors with signal processing are necessary if, for instance, an angular resolution of 1.degree. is desired all around. The alternatives seem to be exacting electronics and a relatively large number of detectors or simpler signal processing, but then for a considerably larger number of detector signals.
If incident illumination can somehow be made to fall along a line, the illuminated position depending on the direction of incidence, it would be possible to use a position sensing detector (PSD) of commercial type. A one-dimensional PSD is an elongate photodetector of special design, in which the generated photocurrent is conducted in two separate paths. The position of the illumination is determined by measuring the currents delivered at the two ends of the PSD and calculations on the basis of these measured values. The relative size of these currents is determined by the difference in internal resistance between the illuminated point and the two current supply points of the PSD. The ratio of the difference to the sum of the two measured currents gives the position along the longitudinal axis of the detector. However, PSD detectors are not intended for so short pulses as those involved in laser warning contexts and, besides, their dynamic range is restricted, which means that commercially available detectors of this type cannot be used for the intended purpose.